This paper presents a comparison of the various methods for combining bimodal wave fatigue damage for flexible risers attached to floating production platforms. Finite Element Analysis is used to determine the riser response, stress ranges, and fatigue damage. The resulting stress ranges or damages are then combined via the various methods outlined in DNV-RP-F204  and the RealLife JIP . A base case of the fatigue damage with swell and sea analyzed together is compared against each method for summation of the damages to determine the level of conservatism in each method. A recommendation of the most appropriate method of combining swell and sea wave damages is presented in this paper.
In the West of Africa region, the seastates are often a combination of mixed swells and seas. The swells are long-period waves generated by large storms in the high latitudes of the South Atlantic. After travelling thousands of miles, waves from these storms reach the West of Africa region as swells with modest heights but with long periods. Shorter period waves of lower height known as seas are generated by local winds. Since swells are generated by storms in the South Atlantic and seas are generated by local winds, there is very little correlation between swells and seas. Since there is little correlation between swells and seas, it is difficult and generally not accurate to group these events together for the purpose of wave fatigue analysis. For this reason, swell and sea waves are usually analyzed in separate sets of dynamic analyses, and the resulting damage from each must be combined to form the total wave fatigue damage. DNV-RP-F204  and the RealLife JIP  suggest that simple summation of the damages via Palmgren-Miner's Rule may be non-conservative.